114 THE HEART AND CIRCULATION. 



where the pressure is greatest, and slowest in the capil- 

 laries, where the area is greatest, the sectional area of 

 the capillaries, known as the peripheral area because it 

 is farthest from the heart, being larger than that of the 

 large arteries. Thus rapidity of flow varies with pres- 

 sure and with area. 



Blood Pressure. Liquids, moreover, are incompres- 

 sible and exert pressure on the walls of the tubes through 

 which they pass. The amount of pressure depends 

 upon the inflow and outflow, increasing directly with the 

 inflow and inversely with the outflow, that is, the smaller 

 the outlet the greater the pressure, and vice versa. The 

 pressure is also greatest nearest to the inflow and grad- 

 ually decreases with distance until at the point of out- 

 flow there is practically no pressure. So, in the arteries 

 the blood pressure is greatest in the large vessels nearer 

 the heart and gradually decreases as they branch into 

 smaller and smaller vessels. In passing through the 

 capillaries, owing to their small size and resultant in- 

 creased friction, the blood meets with more resistance, 

 the peripheral resistance, and this resistance usually 

 regulates the pressure in the arteries. The greater 

 the peripheral resistance, as a rule, the greater the ar- 

 terial pressure. The pressure in the capillaries is very 

 slight and in the veins there is practically no pressure. 

 In fact, in the large veins near the heart the pressure 

 is negative and the blood is almost sucked into the 

 heart. 



Pressure, then, is greatest in the arteries and least in 

 the veins, while the rate of flow is fastest in the arteries 

 300 to 500 millimeters a second and slowest in the 

 capillaries 75 millimeters a second being a little 

 faster again in the veins 200 millimeters a second. 



Blood pressure is gauged by opening a vessel and in- 

 serting a manometer, the pressure being determined 

 by the height to which the mercury is raised. In man 

 the pressure in the arteries is 120 to 160 millimeters. 

 It is considerably heightened during inspiration by 



